1. Field of the Invention
This invention is directed to compositions and to a method. More specifically, this invention concerns itself with novel impact modifiers for rigid polymers, the formulation of rigid polymers containing novel impact modifiers and to a method for enhancement of the impact resistance of rigid polymers.
2. Description of the Prior Art
The modification of the physical properties of polymeric materials and articles prepared therefrom has traditionally been achieved through control of the average molecular weight of the resin, subsequent curing of the resinous material so as to initiate cross-linking thereof, or by the addition of substances, which are at least partially phase compatible (hereinafter referred to as "modifiers") to the polymer matrix. Where the latter avenue is followed, the modifying substance can be associated with the polymer by chemically bonding thereto or by simply intimately blending the modifying substance into the polymer. The modifying substance will ordinarily be selected because it possesses certain properties in which the polymer is deficient. The extent of modification of the polymer will generally be dependent upon the relative concentration of modifier to polymer in the resinous formulation. Where enhancement of one or more of the polymer properties is attempted by the inclusion of modifiers, one or more of the intrinsic properties of the host polymer may be adversely affected. For example, it is widely recognized that attempts in enhancement of the impact resistance of the polymers through the use of modifiers can adversely alter the tensile strength, clarity, resistance to chemical degradation and the resistance of the polymer to photodegradation, as well as result in depression of its heat distortion temperature. The above potential adverse consequences have not discouraged modification of polymeric resins by the inclusion of a variety of such agents. However, an awareness of the potential problems inherent in the use of such modifiers has resulted in an appreciation of the somewhat "empirical" character of the research in this area. This is evidenced by the rather specific combinations of materials disclosed in the prior art as impact modifiers for PVC (see U.S. Pat. Nos. 3,562,235; 3,661,994; 3,655,825; 3,655,826; 3,760,035; 3,763,279; and 4,026,970); for ABS (see U.S. Pat. No. 3,879,496); for the acrylates (see U.S. Pat. No. 3,562,235--previously referenced); and for a variety of other well known polymers and copolymers. As is apparent from the above patent disclosures, the impact modifiers disclosed therein are composite particles having a plurality of distinctly layered components.
Many of the above composite particles can be prepared by well known emulsion and/or suspension polymerization techniques. This is readily achieved by the initiation of polymerization of a seed monomer by well known emulsion polymerization techniques. The polymerization of the seed monomer is accomplished under conditions designed for strict control of the particle size of the seed. Subsequent to formation of the seed monomer into discrete polymer particles in the aqueous emulsion, a second monomer or mixture of monomers can be added thereto and overpolymerized on the seed partiecles; (see U.S. Pat. No. 2,520,259). As taught in U.S. Pat. No. 2,520,259, the newly added monomer can be directively overpolymerized, or caused to be deposited upon the seed latex particles to the substantial exclusion of new particle formation by means of control of emulsifier coverage on the growing latex particles. This second monomer, which is added to and polymerized upon the seed latex particle, can be a rubbery acrylate polymer such as that disclosed in U.S. Pat. No. 3,251,904. Still yet, a third monomer or combination of monomers can be further polymerized upon the rubbery phase of suspended polymer particles using analogous techniques and equipment. The materials used in each of the distinct phases of the toughener, prepared as described hereinabove, are selected not only for their individual properties, but for their compatibility with each of the materials in the distinct phases of the resultant composite particle. Moreover, the outermost portion of the particle is selected for its compatibility with the host or matrix polymer within which the toughener is to be dispersed.
As noted hereinabove, at the core of each such composite particle is a seed which can be prepared from a variety of materials depending upon the properties desired for the ultimate composite particles. Seeded tougheners are disclosed in the patent literature wherein the seed has a Tg in excess of room temperature (e.g. U.S. Pat. Nos. 3,661,994 and 4,026,970). The patent literature also discloses "unseeded" impact modifiers comprising a plurality of separate and distinct phases (e.g. U.S. Pat. Nos. 3,562,253; 3,655,825; and 3,655,826). In such "unseeded" particles, the outer shell is formed in two separate stages, thus given the appearances of a three phase system although only containing essentially two functional distinctive phases.
Impact modifiers (also hereinafter referred to as "tougheners") wherein the seed has a Tg substantially below room temperature will be referred to throughout the balance of this disclosure as "soft-seeded tougheners"; whereas, an impact modifier wherein the seed has a Tg in excess of room temperature will be referred to throughout the balance of this disclosure as "hard-seeded tougheners".
Notwithstanding the enhancement in impact resistance of rigid polymers by the inclusion of one or more of the above modifying agents, further improvement is still required. As indicated previously, impact modifiers, as well as other modifying agents, often adversely alter the intrinsic physical properties of polymeric materials even when only present therein at relatively low concentrations. Thus, there is a continuing need for impact modifiers which are effective at increasing the toughness of the host polymer and yet can efficiently achieve such a result at concentrations having a minimum of adverse impact upon the host resin.